The human and animal trypanosomiases have major medical, veterinary, social, economic and nutritional consequences throughout equatorial Africa. The salivarian trypanosomes have developed a unique mechanism for antigenic variation, which enables them to avoid elimination by the host's immune responses. Each trypanosome is covered by a surface coat consisting of a closely packed layer of about 10 million molecules of, essentially, a single molecular species of a large family of variant surface glycoproteins (VSGs). Antigenic variation occurs by gene rearrangements which regulate the sequential expression of 100-1,000 individual genes encoding antigenically distinct VSGs. The mechanisms regulating gene expression form one of the central themes of molecular biology today. Elucidation of mechanisms regulating antigenic variation in trypanosomes may provide novel insights pertinent to other cell systems, just as other systems suggest possible mechanisms and experimental approaches to use in trypanosomes. The objective of this proposal is detailed analysis of the mechanisms regulating (VSG) gene expression in Trypanosoma brucei. The proposed research will refine our understanding of the relationships between gene rearrangement and VSG expression, by characterizing the environment of the expressed gene in greater detail than hitherto. The role of the 'mini-exon' in trypanosome gene expression will be studied. Exhaustive investigations are planned to develop systems whereby specific DNA sequences can be introduced into trypanosomes, facilitating the introduction of genetic analysis as a tool that will be essential to a complete understanding of antigenic variation. Basic data on the rate of VSG switching, its dependence on the chromosomal environment of incoming and outgoing genes, and factors determining the sequence of VSG gene expression will be sought. The results will provide a firmer basis for assessing the prospects for control of trypanosomiasis.